<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>perltrap - Perl traps for the unwary - Perldoc Browser</title>
    <link rel="search" href="https://perldoc.perl.org/opensearch.xml" type="application/opensearchdescription+xml" title="Perldoc Browser">
    <link rel="canonical" href="https://perldoc.perl.org/perltrap">
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css" integrity="sha384-JcKb8q3iqJ61gNV9KGb8thSsNjpSL0n8PARn9HuZOnIxN0hoP+VmmDGMN5t9UJ0Z" crossorigin="anonymous">
    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/10.5.0/styles/stackoverflow-light.min.css" integrity="sha512-cG1IdFxqipi3gqLmksLtuk13C+hBa57a6zpWxMeoY3Q9O6ooFxq50DayCdm0QrDgZjMUn23z/0PMZlgft7Yp5Q==" crossorigin="anonymous" />
    <style>
      body {
        background: #f4f4f5;
        color: #020202;
      }
      .navbar-dark {
        background-image: -webkit-linear-gradient(top, #005f85 0, #002e49 100%);
        background-image: -o-linear-gradient(top, #005f85 0, #002e49 100%);
        background-image: linear-gradient(to bottom, #005f85 0, #002e49 100%);
        filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ff005f85', endColorstr='#ff002e49', GradientType=0);
        background-repeat: repeat-x;
      }
      .navbar-dark .navbar-nav .nav-link,
      .navbar-dark .navbar-nav .nav-link:focus { color: #fff }
      .navbar-dark .navbar-nav .nav-link:hover { color: #ffef68 }
      #wrapperlicious {
        margin: 0 auto;
        font: 0.9em 'Helvetica Neue', Helvetica, sans-serif;
        font-weight: normal;
        line-height: 1.5em;
        margin: 0;
        padding: 0;
      }
      #wrapperlicious h1 { font-size: 1.5em }
      #wrapperlicious h2 { font-size: 1.3em }
      #wrapperlicious h3 { font-size: 1.1em }
      #wrapperlicious h4 { font-size: 0.9em }
      #wrapperlicious h1, #wrapperlicious h2, #wrapperlicious h3,
      #wrapperlicious h4, #wrapperlicious dt {
        color: #020202;
        margin-top: 1em;
        margin-bottom: 1em;
        position: relative;
        font-weight: bold;
      }
      #wrapperlicious a { color: inherit; text-decoration: underline }
      #wrapperlicious #toc { text-decoration: none }
      #wrapperlicious a:hover { color: #2a2a2a }
      #wrapperlicious a img { border: 0 }
      #wrapperlicious :not(pre) > code {
        color: inherit;
        background-color: rgba(0, 0, 0, 0.04);
        border-radius: 3px;
        font: 0.9em Consolas, Menlo, Monaco, monospace;
        padding: 0.3em;
      }
      #wrapperlicious dd {
        margin: 0;
        margin-left: 2em;
      }
      #wrapperlicious dt {
        color: #2a2a2a;
        font-weight: bold;
        margin-left: 0.9em;
      }
      #wrapperlicious p {
        margin-bottom: 1em;
        margin-top: 1em;
      }
      #wrapperlicious li > p {
        margin-bottom: 0;
        margin-top: 0;
      }
      #wrapperlicious pre {
        border: 1px solid #c1c1c1;
        border-radius: 3px;
        font: 100% Consolas, Menlo, Monaco, monospace;
        margin-bottom: 1em;
        margin-top: 1em;
      }
      #wrapperlicious pre > code {
        display: block;
        background-color: #f6f6f6;
        font: 0.9em Consolas, Menlo, Monaco, monospace;
        line-height: 1.5em;
        text-align: left;
        white-space: pre;
        padding: 1em;
      }
      #wrapperlicious dl, #wrapperlicious ol, #wrapperlicious ul {
        margin-bottom: 1em;
        margin-top: 1em;
      }
      #wrapperlicious ul {
        list-style-type: square;
      }
      #wrapperlicious ul ul {
        margin-bottom: 0px;
        margin-top: 0px;
      }
      #footer {
        font-size: 0.8em;
        padding-top: 0.5em;
        text-align: center;
      }
      #more {
        display: inline;
        font-size: 0.8em;
      }
      #perldocdiv {
        background-color: #fff;
        border: 1px solid #c1c1c1;
        border-bottom-left-radius: 5px;
        border-bottom-right-radius: 5px;
        margin-left: auto;
        margin-right: auto;
        padding: 3em;
        padding-top: 1em;
        max-width: 960px;
      }
      #moduleversion { float: right }
      #wrapperlicious .leading-notice {
        font-style: italic;
        padding-left: 1em;
        margin-top: 1em;
        margin-bottom: 1em;
      }
      #wrapperlicious .permalink {
        display: none;
        left: -0.75em;
        position: absolute;
        padding-right: 0.25em;
        text-decoration: none;
      }
      #wrapperlicious h1:hover .permalink, #wrapperlicious h2:hover .permalink,
      #wrapperlicious h3:hover .permalink, #wrapperlicious h4:hover .permalink,
      #wrapperlicious dt:hover .permalink {
        display: block;
      }
    </style>
    <!-- Global site tag (gtag.js) - Google Analytics -->
    <script async src="https://www.googletagmanager.com/gtag/js?id=G-KVNWBNT5FB"></script>
    <script>
      window.dataLayer = window.dataLayer || [];
      function gtag(){dataLayer.push(arguments);}
      gtag('js', new Date());

      gtag('config', 'G-KVNWBNT5FB');
      gtag('config', 'UA-50555-3');
    </script>
  </head>
  <body>
    <nav class="navbar navbar-expand-md navbar-dark bg-dark justify-content-between">
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <a class="navbar-brand" href="https://perldoc.perl.org/">Perldoc Browser</a>
  <div class="collapse navbar-collapse" id="navbarNav">
    <ul class="navbar-nav mr-auto">
      <li class="nav-item dropdown text-nowrap">
        <a class="nav-link dropdown-toggle" href="perltrap.html#" id="dropdownlink-stable" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">5.35.0</a>
        <div class="dropdown-menu" aria-labelledby="dropdownlink-stable">
          <a class="dropdown-item" href="https://perldoc.perl.org/perltrap">Latest</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.38.2/perltrap">5.38.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.38.1/perltrap">5.38.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.38.0/perltrap">5.38.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.36.3/perltrap">5.36.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.36.2/perltrap">5.36.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.36.1/perltrap">5.36.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.36.0/perltrap">5.36.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.34.3/perltrap">5.34.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.34.2/perltrap">5.34.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.34.1/perltrap">5.34.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.34.0/perltrap">5.34.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.32.1/perltrap">5.32.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.32.0/perltrap">5.32.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.30.3/perltrap">5.30.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.30.2/perltrap">5.30.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.30.1/perltrap">5.30.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.30.0/perltrap">5.30.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.28.3/perltrap">5.28.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.28.2/perltrap">5.28.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.28.1/perltrap">5.28.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.28.0/perltrap">5.28.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.26.3/perltrap">5.26.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.26.2/perltrap">5.26.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.26.1/perltrap">5.26.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.26.0/perltrap">5.26.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.24.4/perltrap">5.24.4</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.24.3/perltrap">5.24.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.24.2/perltrap">5.24.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.24.1/perltrap">5.24.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.24.0/perltrap">5.24.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.22.4/perltrap">5.22.4</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.22.3/perltrap">5.22.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.22.2/perltrap">5.22.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.22.1/perltrap">5.22.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.22.0/perltrap">5.22.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.20.3/perltrap">5.20.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.20.2/perltrap">5.20.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.20.1/perltrap">5.20.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.20.0/perltrap">5.20.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.18.4/perltrap">5.18.4</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.18.3/perltrap">5.18.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.18.2/perltrap">5.18.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.18.1/perltrap">5.18.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.18.0/perltrap">5.18.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.16.3/perltrap">5.16.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.16.2/perltrap">5.16.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.16.1/perltrap">5.16.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.16.0/perltrap">5.16.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.14.4/perltrap">5.14.4</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.14.3/perltrap">5.14.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.14.2/perltrap">5.14.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.14.1/perltrap">5.14.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.14.0/perltrap">5.14.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.12.5/perltrap">5.12.5</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.12.4/perltrap">5.12.4</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.12.3/perltrap">5.12.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.12.2/perltrap">5.12.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.12.1/perltrap">5.12.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.12.0/perltrap">5.12.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.10.1/perltrap">5.10.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.10.0/perltrap">5.10.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.9/perltrap">5.8.9</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.8/perltrap">5.8.8</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.7/perltrap">5.8.7</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.6/perltrap">5.8.6</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.5/perltrap">5.8.5</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.4/perltrap">5.8.4</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.3/perltrap">5.8.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.2/perltrap">5.8.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.1/perltrap">5.8.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.8.0/perltrap">5.8.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.6.2/perltrap">5.6.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.6.1/perltrap">5.6.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.6.0/perltrap">5.6.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.005_04/perltrap">5.005_04</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.005_03/perltrap">5.005_03</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.005_02/perltrap">5.005_02</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.005_01/perltrap">5.005_01</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.005/perltrap">5.005</a>
        </div>
      </li>
      <li class="nav-item dropdown text-nowrap">
        <a class="nav-link dropdown-toggle" href="perltrap.html#" id="dropdownlink-dev" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dev</a>
        <div class="dropdown-menu" aria-labelledby="dropdownlink-dev">
          <a class="dropdown-item" href="https://perldoc.perl.org/blead/perltrap">blead</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.39.6/perltrap">5.39.6</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.39.5/perltrap">5.39.5</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.39.4/perltrap">5.39.4</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.39.3/perltrap">5.39.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.39.2/perltrap">5.39.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.39.1/perltrap">5.39.1</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.38.0-RC2/perltrap">5.38.0-RC2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.38.0-RC1/perltrap">5.38.0-RC1</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.11/perltrap">5.37.11</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.10/perltrap">5.37.10</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.9/perltrap">5.37.9</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.8/perltrap">5.37.8</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.7/perltrap">5.37.7</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.6/perltrap">5.37.6</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.5/perltrap">5.37.5</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.4/perltrap">5.37.4</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.3/perltrap">5.37.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.2/perltrap">5.37.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.1/perltrap">5.37.1</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.37.0/perltrap">5.37.0</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.11/perltrap">5.35.11</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.10/perltrap">5.35.10</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.9/perltrap">5.35.9</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.8/perltrap">5.35.8</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.7/perltrap">5.35.7</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.6/perltrap">5.35.6</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.5/perltrap">5.35.5</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.4/perltrap">5.35.4</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.3/perltrap">5.35.3</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.2/perltrap">5.35.2</a>
          <a class="dropdown-item" href="https://perldoc.perl.org/5.35.1/perltrap">5.35.1</a>
          <a class="dropdown-item active" href="perltrap.html">5.35.0</a>
        </div>
      </li>
      <li class="nav-item dropdown text-nowrap">
        <a class="nav-link dropdown-toggle" href="perltrap.html#" id="dropdownlink-nav" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Documentation</a>
        <div class="dropdown-menu" aria-labelledby="dropdownlink-nav">
          <a class="dropdown-item" href="perl.html">Perl</a>
          <a class="dropdown-item" href="perlintro.html">Intro</a>
          <a class="dropdown-item" href="perl.html#Tutorials">Tutorials</a>
          <a class="dropdown-item" href="perlfaq.html">FAQs</a>
          <a class="dropdown-item" href="perl.html#Reference-Manual">Reference</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="perlop.html">Operators</a>
          <a class="dropdown-item" href="functions.html">Functions</a>
          <a class="dropdown-item" href="variables.html">Variables</a>
          <a class="dropdown-item" href="modules.html">Modules</a>
          <a class="dropdown-item" href="perlutil.html">Utilities</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="perlcommunity.html">Community</a>
          <a class="dropdown-item" href="perlhist.html">History</a>
        </div>
      </li>
    </ul>
    <ul class="navbar-nav">
      <script>
        function set_expand (expand) {
          var perldocdiv = document.getElementById('perldocdiv');
          var width = window.getComputedStyle(perldocdiv).getPropertyValue('max-width');
          var expanded = (width == '' || width == 'none') ? true : false;
          if (expand === null) {
            expand = !expanded;
          }
          if ((expand && !expanded) || (!expand && expanded)) {
            perldocdiv.style.setProperty('max-width', expand ? 'none' : '960px');
            var button_classlist = document.getElementById('content-expand-button').classList;
            if (expand) {
              button_classlist.add('btn-light');
              button_classlist.remove('btn-outline-light');
            } else {
              button_classlist.add('btn-outline-light');
              button_classlist.remove('btn-light');
            }
          }
          return expand;
        }
        function toggle_expand () {
          var expand = set_expand(null);
          document.cookie = 'perldoc_expand=' + (expand ? 1 : 0) + '; path=/; expires=Tue, 19 Jan 2038 03:14:07 UTC';
        }
        function read_expand () {
          return document.cookie.split(';').some(function (item) { return item.indexOf('perldoc_expand=1') >= 0 });
        }
        if (document.readyState === 'loading') {
          document.addEventListener('DOMContentLoaded', function () {
            if (read_expand()) {
              set_expand(true);
            }
          });
        } else if (read_expand()) {
          set_expand(true);
        }
      </script>
      <button id="content-expand-button" type="button" class="btn btn-outline-light d-none d-lg-inline-block mr-2" onclick="toggle_expand()">Expand</button>
    <ul>
    <form class="form-inline" method="get" action="https://perldoc.perl.org/5.35.0/search">
      <input class="form-control mr-3" type="search" name="q" placeholder="Search" aria-label="Search">
    </form>
  </div>
</nav>

    <div id="wrapperlicious" class="container-fluid">
      <div id="perldocdiv">
        <div id="links">
          <a href="perltrap.html">perltrap</a>
          <div id="more">
            (<a href="perltrap.txt">source</a>,
            <a href="https://metacpan.org/pod/perltrap">CPAN</a>)
          </div>
        </div>
        <div class="leading-notice">
          You are viewing the version of this documentation from Perl 5.35.0.
            This is a development version of Perl.
        </div>
        <h1><a id="toc">CONTENTS</a></h1>
                  <ul>
              <li>
                <a class="text-decoration-none" href="perltrap.html#NAME">NAME</a>
              </li>
              <li>
                <a class="text-decoration-none" href="perltrap.html#DESCRIPTION">DESCRIPTION</a>
                            <ul>
              <li>
                <a class="text-decoration-none" href="perltrap.html#Awk-Traps">Awk Traps</a>
              </li>
              <li>
                <a class="text-decoration-none" href="perltrap.html#C/C++-Traps">C/C++ Traps</a>
              </li>
              <li>
                <a class="text-decoration-none" href="perltrap.html#JavaScript-Traps">JavaScript Traps</a>
              </li>
              <li>
                <a class="text-decoration-none" href="perltrap.html#Sed-Traps">Sed Traps</a>
              </li>
              <li>
                <a class="text-decoration-none" href="perltrap.html#Shell-Traps">Shell Traps</a>
              </li>
              <li>
                <a class="text-decoration-none" href="perltrap.html#Perl-Traps">Perl Traps</a>
              </li>
          </ul>

              </li>
          </ul>

      <h1 id="NAME"><a class="permalink" href="perltrap.html#NAME">#</a>NAME</h1>

<p>perltrap - Perl traps for the unwary</p>

<h1 id="DESCRIPTION"><a class="permalink" href="perltrap.html#DESCRIPTION">#</a>DESCRIPTION</h1>

<p>The biggest trap of all is forgetting to <code>use warnings</code> or use the <b>-w</b> switch; see <a href="warnings.html">warnings</a> and <a href="perlrun.html#-w">&quot;-w&quot; in perlrun</a>. The second biggest trap is not making your entire program runnable under <code>use strict</code>. The third biggest trap is not reading the list of changes in this version of Perl; see <a href="perldelta.html">perldelta</a>.</p>

<h2 id="Awk-Traps"><a class="permalink" href="perltrap.html#Awk-Traps">#</a><a id="Awk"></a>Awk Traps</h2>

<p>Accustomed <b>awk</b> users should take special note of the following:</p>

<ul>

<li><p>A Perl program executes only once, not once for each input line. You can do an implicit loop with <code>-n</code> or <code>-p</code>.</p>

</li>
<li><p>The English module, loaded via</p>

<pre><code>use English;</code></pre>

<p>allows you to refer to special variables (like <code>$/</code>) with names (like $RS), as though they were in <b>awk</b>; see <a href="perlvar.html">perlvar</a> for details.</p>

</li>
<li><p>Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.</p>

</li>
<li><p>Curly brackets are required on <code>if</code>s and <code>while</code>s.</p>

</li>
<li><p>Variables begin with &quot;$&quot;, &quot;@&quot; or &quot;%&quot; in Perl.</p>

</li>
<li><p>Arrays index from 0. Likewise string positions in substr() and index().</p>

</li>
<li><p>You have to decide whether your array has numeric or string indices.</p>

</li>
<li><p>Hash values do not spring into existence upon mere reference.</p>

</li>
<li><p>You have to decide whether you want to use string or numeric comparisons.</p>

</li>
<li><p>Reading an input line does not split it for you. You get to split it to an array yourself. And the split() operator has different arguments than <b>awk</b>&#39;s.</p>

</li>
<li><p>The current input line is normally in $_, not $0. It generally does not have the newline stripped. ($0 is the name of the program executed.) See <a href="perlvar.html">perlvar</a>.</p>

</li>
<li><p>$&lt;<i>digit</i>&gt; does not refer to fields--it refers to substrings matched by the last match pattern.</p>

</li>
<li><p>The print() statement does not add field and record separators unless you set <code>$,</code> and <code>$\</code>. You can set $OFS and $ORS if you&#39;re using the English module.</p>

</li>
<li><p>You must open your files before you print to them.</p>

</li>
<li><p>The range operator is &quot;..&quot;, not comma. The comma operator works as in C.</p>

</li>
<li><p>The match operator is &quot;=~&quot;, not &quot;~&quot;. (&quot;~&quot; is the one&#39;s complement operator, as in C.)</p>

</li>
<li><p>The exponentiation operator is &quot;**&quot;, not &quot;^&quot;. &quot;^&quot; is the XOR operator, as in C. (You know, one could get the feeling that <b>awk</b> is basically incompatible with C.)</p>

</li>
<li><p>The concatenation operator is &quot;.&quot;, not the null string. (Using the null string would render <code>/pat/ /pat/</code> unparsable, because the third slash would be interpreted as a division operator--the tokenizer is in fact slightly context sensitive for operators like &quot;/&quot;, &quot;?&quot;, and &quot;&gt;&quot;. And in fact, &quot;.&quot; itself can be the beginning of a number.)</p>

</li>
<li><p>The <code>next</code>, <code>exit</code>, and <code>continue</code> keywords work differently.</p>

</li>
<li><p>The following variables work differently:</p>

<pre><code>Awk	Perl
ARGC	scalar @ARGV (compare with $#ARGV)
ARGV[0]	$0
FILENAME	$ARGV
FNR	$. - something
FS	(whatever you like)
NF	$#Fld, or some such
NR	$.
OFMT	$#
OFS	$,
ORS	$\
RLENGTH	length($&amp;)
RS	$/
RSTART	length($`)
SUBSEP	$;</code></pre>

</li>
<li><p>You cannot set $RS to a pattern, only a string.</p>

</li>
<li><p>When in doubt, run the <b>awk</b> construct through <b>a2p</b> and see what it gives you.</p>

</li>
</ul>

<h2 id="C/C++-Traps"><a class="permalink" href="perltrap.html#C/C++-Traps">#</a><a id="C"></a><a id="C-C-Traps"></a>C/C++ Traps</h2>

<p>Cerebral C and C++ programmers should take note of the following:</p>

<ul>

<li><p>Curly brackets are required on <code>if</code>&#39;s and <code>while</code>&#39;s.</p>

</li>
<li><p>You must use <code>elsif</code> rather than <code>else if</code>.</p>

</li>
<li><p>The <code>break</code> and <code>continue</code> keywords from C become in Perl <code>last</code> and <code>next</code>, respectively. Unlike in C, these do <i>not</i> work within a <code>do { } while</code> construct. See <a href="perlsyn.html#Loop-Control">&quot;Loop Control&quot; in perlsyn</a>.</p>

</li>
<li><p>The switch statement is called <code>given</code>/<code>when</code> and only available in perl 5.10 or newer. See <a href="perlsyn.html#Switch-Statements">&quot;Switch Statements&quot; in perlsyn</a>.</p>

</li>
<li><p>Variables begin with &quot;$&quot;, &quot;@&quot; or &quot;%&quot; in Perl.</p>

</li>
<li><p>Comments begin with &quot;#&quot;, not &quot;/*&quot; or &quot;//&quot;. Perl may interpret C/C++ comments as division operators, unterminated regular expressions or the defined-or operator.</p>

</li>
<li><p>You can&#39;t take the address of anything, although a similar operator in Perl is the backslash, which creates a reference.</p>

</li>
<li><p><code>ARGV</code> must be capitalized. <code>$ARGV[0]</code> is C&#39;s <code>argv[1]</code>, and <code>argv[0]</code> ends up in <code>$0</code>.</p>

</li>
<li><p>System calls such as link(), unlink(), rename(), etc. return nonzero for success, not 0. (system(), however, returns zero for success.)</p>

</li>
<li><p>Signal handlers deal with signal names, not numbers. Use <code>kill -l</code> to find their names on your system.</p>

</li>
</ul>

<h2 id="JavaScript-Traps"><a class="permalink" href="perltrap.html#JavaScript-Traps">#</a><a id="JavaScript"></a>JavaScript Traps</h2>

<p>Judicious JavaScript programmers should take note of the following:</p>

<ul>

<li><p>In Perl, binary <code>+</code> is always addition. <code>$string1 + $string2</code> converts both strings to numbers and then adds them. To concatenate two strings, use the <code>.</code> operator.</p>

</li>
<li><p>The <code>+</code> unary operator doesn&#39;t do anything in Perl. It exists to avoid syntactic ambiguities.</p>

</li>
<li><p>Unlike <code>for...in</code>, Perl&#39;s <code>for</code> (also spelled <code>foreach</code>) does not allow the left-hand side to be an arbitrary expression. It must be a variable:</p>

<pre><code>   for my $variable (keys %hash) {
	...
   }</code></pre>

<p>Furthermore, don&#39;t forget the <code>keys</code> in there, as <code>foreach my $kv (%hash) {}</code> iterates over the keys and values, and is generally not useful ($kv would be a key, then a value, and so on).</p>

</li>
<li><p>To iterate over the indices of an array, use <code>foreach my $i (0 .. $#array) {}</code>. <code>foreach my $v (@array) {}</code> iterates over the values.</p>

</li>
<li><p>Perl requires braces following <code>if</code>, <code>while</code>, <code>foreach</code>, etc.</p>

</li>
<li><p>In Perl, <code>else if</code> is spelled <code>elsif</code>.</p>

</li>
<li><p><code>? :</code> has higher precedence than assignment. In JavaScript, one can write:</p>

<pre><code class="plaintext">condition ? do_something() : variable = 3</code></pre>

<p>and the variable is only assigned if the condition is false. In Perl, you need parentheses:</p>

<pre><code>$condition ? do_something() : ($variable = 3);</code></pre>

<p>Or just use <code>if</code>.</p>

</li>
<li><p>Perl requires semicolons to separate statements.</p>

</li>
<li><p>Variables declared with <code>my</code> only affect code <i>after</i> the declaration. You cannot write <code>$x = 1; my $x;</code> and expect the first assignment to affect the same variable. It will instead assign to an <code>$x</code> declared previously in an outer scope, or to a global variable.</p>

<p>Note also that the variable is not visible until the following <i>statement</i>. This means that in <code>my $x = 1 + $x</code> the second $x refers to one declared previously.</p>

</li>
<li><p><code>my</code> variables are scoped to the current block, not to the current function. If you write <code>{my $x;} $x;</code>, the second <code>$x</code> does not refer to the one declared inside the block.</p>

</li>
<li><p>An object&#39;s members cannot be made accessible as variables. The closest Perl equivalent to <code>with(object) { method() }</code> is <code>for</code>, which can alias <code>$_</code> to the object:</p>

<pre><code>    for ($object) {
	$_-&gt;method;
    }</code></pre>

</li>
<li><p>The object or class on which a method is called is passed as one of the method&#39;s arguments, not as a separate <code>this</code> value.</p>

</li>
</ul>

<h2 id="Sed-Traps"><a class="permalink" href="perltrap.html#Sed-Traps">#</a><a id="Sed"></a>Sed Traps</h2>

<p>Seasoned <b>sed</b> programmers should take note of the following:</p>

<ul>

<li><p>A Perl program executes only once, not once for each input line. You can do an implicit loop with <code>-n</code> or <code>-p</code>.</p>

</li>
<li><p>Backreferences in substitutions use &quot;$&quot; rather than &quot;\&quot;.</p>

</li>
<li><p>The pattern matching metacharacters &quot;(&quot;, &quot;)&quot;, and &quot;|&quot; do not have backslashes in front.</p>

</li>
<li><p>The range operator is <code>...</code>, rather than comma.</p>

</li>
</ul>

<h2 id="Shell-Traps"><a class="permalink" href="perltrap.html#Shell-Traps">#</a><a id="Shell"></a>Shell Traps</h2>

<p>Sharp shell programmers should take note of the following:</p>

<ul>

<li><p>The backtick operator does variable interpolation without regard to the presence of single quotes in the command.</p>

</li>
<li><p>The backtick operator does no translation of the return value, unlike <b>csh</b>.</p>

</li>
<li><p>Shells (especially <b>csh</b>) do several levels of substitution on each command line. Perl does substitution in only certain constructs such as double quotes, backticks, angle brackets, and search patterns.</p>

</li>
<li><p>Shells interpret scripts a little bit at a time. Perl compiles the entire program before executing it (except for <code>BEGIN</code> blocks, which execute at compile time).</p>

</li>
<li><p>The arguments are available via @ARGV, not $1, $2, etc.</p>

</li>
<li><p>The environment is not automatically made available as separate scalar variables.</p>

</li>
<li><p>The shell&#39;s <code>test</code> uses &quot;=&quot;, &quot;!=&quot;, &quot;&lt;&quot; etc for string comparisons and &quot;-eq&quot;, &quot;-ne&quot;, &quot;-lt&quot; etc for numeric comparisons. This is the reverse of Perl, which uses <code>eq</code>, <code>ne</code>, <code>lt</code> for string comparisons, and <code>==</code>, <code>!=</code> <code>&lt;</code> etc for numeric comparisons.</p>

</li>
</ul>

<h2 id="Perl-Traps"><a class="permalink" href="perltrap.html#Perl-Traps">#</a><a id="Perl"></a>Perl Traps</h2>

<p>Practicing Perl Programmers should take note of the following:</p>

<ul>

<li><p>Remember that many operations behave differently in a list context than they do in a scalar one. See <a href="perldata.html">perldata</a> for details.</p>

</li>
<li><p>Avoid barewords if you can, especially all lowercase ones. You can&#39;t tell by just looking at it whether a bareword is a function or a string. By using quotes on strings and parentheses on function calls, you won&#39;t ever get them confused.</p>

</li>
<li><p>You cannot discern from mere inspection which builtins are unary operators (like chop() and chdir()) and which are list operators (like print() and unlink()). (Unless prototyped, user-defined subroutines can <b>only</b> be list operators, never unary ones.) See <a href="perlop.html">perlop</a> and <a href="perlsub.html">perlsub</a>.</p>

</li>
<li><p>People have a hard time remembering that some functions default to $_, or @ARGV, or whatever, but that others which you might expect to do not.</p>

</li>
<li><p>The &lt;FH&gt; construct is not the name of the filehandle, it is a readline operation on that handle. The data read is assigned to $_ only if the file read is the sole condition in a while loop:</p>

<pre><code>while (&lt;FH&gt;)      { }
while (defined($_ = &lt;FH&gt;)) { }..
&lt;FH&gt;;  # data discarded!</code></pre>

</li>
<li><p>Remember not to use <code>=</code> when you need <code>=~</code>; these two constructs are quite different:</p>

<pre><code>$x =  /foo/;
$x =~ /foo/;</code></pre>

</li>
<li><p>The <code>do {}</code> construct isn&#39;t a real loop that you can use loop control on.</p>

</li>
<li><p>Use <code>my()</code> for local variables whenever you can get away with it (but see <a href="perlform.html">perlform</a> for where you can&#39;t). Using <code>local()</code> actually gives a local value to a global variable, which leaves you open to unforeseen side-effects of dynamic scoping.</p>

</li>
<li><p>If you localize an exported variable in a module, its exported value will not change. The local name becomes an alias to a new value but the external name is still an alias for the original.</p>

</li>
</ul>

<p>As always, if any of these are ever officially declared as bugs, they&#39;ll be fixed and removed.</p>


      </div>
      <div id="footer">
        <p>Perldoc Browser is maintained by Dan Book (<a href="https://metacpan.org/author/DBOOK">DBOOK</a>). Please contact him via the <a href="https://github.com/Grinnz/perldoc-browser/issues">GitHub issue tracker</a> or <a href="mailto:dbook@cpan.org">email</a> regarding any issues with the site itself, search, or rendering of documentation.</p>

<p>The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the <a href="https://github.com/Perl/perl5/issues">Perl issue tracker</a>, the <a href="https://lists.perl.org/list/perl5-porters.html">mailing list</a>, or <a href="https://kiwiirc.com/client/irc.perl.org/p5p">IRC</a> to report any issues with the contents or format of the documentation.</p>


      </div>
    </div>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.slim.min.js" integrity="sha512-/DXTXr6nQodMUiq+IUJYCt2PPOUjrHJ9wFrqpJ3XkgPNOZVfMok7cRw6CSxyCQxXn6ozlESsSh1/sMCTF1rL/g==" crossorigin="anonymous"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.1/umd/popper.min.js" integrity="sha512-ubuT8Z88WxezgSqf3RLuNi5lmjstiJcyezx34yIU2gAHonIi27Na7atqzUZCOoY4CExaoFumzOsFQ2Ch+I/HCw==" crossorigin="anonymous"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js" integrity="sha384-B4gt1jrGC7Jh4AgTPSdUtOBvfO8shuf57BaghqFfPlYxofvL8/KUEfYiJOMMV+rV" crossorigin="anonymous"></script>
    <script src="../highlight.pack.js"></script>
    <script>hljs.highlightAll();</script>
  </body>
</html>
